The inventive subject matter relates to power conversion apparatus and methods and, more particularly, to uninterruptible power supply (UPS) apparatus and methods.
UPS systems are commonly used in installations such as data centers, medical centers and industrial facilities. UPS systems may be used in such installations to provide backup power to maintain operation in event of failure of the primary utility supply. These UPS systems often have an “on-line” configuration including a rectifier and inverter coupled by a DC link that is also coupled to an auxiliary power source, such as a battery, fuel cell or other energy storage device. Other configurations, such as standby and line-interactive configurations, may also be used. UPS systems may have a modular structure including two or more UPS modules, each of which may include, for example, a rectifier, an inverter and a DC/DC converter for interfacing to a battery or other DC power source. The modules commonly are designed to operate in parallel to provide scalable power capacity, e.g., the modules may be coupled in common to an AC source, a DC source (e.g., a battery) and/or a load.
Power quality is generally a concern for electrical power users, particularly those involved with critical power applications, such as data center applications. Data center applications have traditionally used large, transformer-based UPS systems. However, increased energy cost, high real estate cost, and environmental concerns associated with transformer-based designs have motivated the development of transformer-less designs. Differences in the technologies used in transformer-based and transformer-less UPS designs may be of concern in particular applications, such in applications using a 3-wire service.
A conventional transformer-less UPS, such as the Eaton® 9395 Power Xpert™ UPS, may include a boost rectifier configured to be coupled to an AC source and a buck inverter that is coupled to the output of the rectifier by a DC bus and that produces an AC output for a load. This arrangement allows operation without an output transformer. A separate DC/DC converter may be used to connect a battery to the DC bus and to regulate variations in DC voltage from the battery. An additional converter may be used to create a neutral by modulating a zero sequence equal to one-third the sum of the three output phases.
The UPS may fed from a source of supply that is referenced to earth ground, such as a source transformer with a wye secondary with its neutral bonded to earth ground. Because the UPS output shares the same plane of reference as the input source, grounding of the UPS output neutral may not be required when the UPS is providing power to the load from the input source. However, if the UPS input source is removed due to, for example, a loss of the utility power, the inverter may be powered from an ungrounded battery source. This may allow the UPS output to “float” with respect to earth ground.
Operation of a transformer-less UPS with an ungrounded output may be undesirable. NEC non-compliance questions and complaints may produce a level of uncertainty for the customer or system designer. When the transformer-less UPS is operating in an on-battery mode, the ungrounded output may not trip downstream over-current protection devices in response to output phase-to-ground faults, which may cause some unwanted system conflicts. For example, if the fault remains after power is restored, downstream breakers may trip, seemingly without warning. An ungrounded output may also give rise to concerns about overvoltage conditions, as it raises the possibility that the output three phase voltage space may shift or move away from ground to levels that may challenge downstream components that are not rated for such voltages. These concerns are generally not associated with transformer-based UPSs, so customers and/or consultants may be hesitant to use transformer-less UPSs.
An approach for avoiding this issue with a transformer-less UPS is to run a neutral conductor from the input source to the inverter neutral point of the transformer-less UPS, thus making the UPS output a grounded system whether on-line or on-battery. This solution may provide acceptable results under most operating conditions but may entail a heavy installation expense associated with running the extra neutral conductor over a relatively long distance.